1. Field of the Invention
The present invention relates to a balun used to mutually convert a unbalanced transmission line and a balanced transmission line, and in particular relates to a balun that is suitable for use in a high-frequency band, such as a microwave band and that attains a wider bandwidth.
2. Description of the Related Art
A balun is known as a transformer for converting from an unbalanced transmission line to a balanced transmission line and vice versa, and is used, for example, in an input/output end of a repeater in a communication system. Various baluns are known, and one of those is a high-frequency balun using a microstrip line (MSL) coupling line, known as an unbalanced high-frequency transmission line. In recent years, in optical communication systems or the like, information has been transmitted by using UWB (Ultra Wide Band) as a frequency band, for example, a frequency band from 3.1 to 10.6 GHz, and with this situation, a wider bandwidth is required for a high-frequency balun.
FIG. 1A shows a conventional high-frequency balun formed by a microstrip line coupling line, and FIG. 1B shows a cross-sectional view taken along a line A-A in FIG. 1A. The high-frequency balun includes unbalanced microstrip line 1 used for unbalanced input/output, and a pair of microstrip lines 2, 3 used for balanced input/output. In microstrip lines 1 to 3, a high-frequency wave component travels or propagates by electromagnetic fields between signal lines 1a, 2a, 3a arranged in one main surface of substrate 4 made of a dielectric material and ground conductor 5 formed over the entirety of the other main surface of substrate 4.
Unbalanced microstrip line 1 is formed, for example, by extending signal line 1a from the left end of substrate 4 in the horizontal direction in drawings. Balanced microstrip lines 2, 3 are formed, for example, by extending a pair of signal lines 2a, 3a from the lower end of substrate 4 to be close each other and to be parallel. Tip portions of signal lines 2a, 3a are bent in directions that are mutually reversed, and each of signal lines 2a, 3a extends along unbalanced microstrip line 1 (i.e., signal line 1a) in parallel. Each tip end of bent portion 2x, 3x in unbalanced microstrip lines 2, 3 (i.e., signal lines 2a, 3a) is electrically connected to ground conductor 5 in the other main surface by electrode through-connection 6, such as a via-hole and a through-hole. Then, each bent portion 2x, 3x has an electric length of λ/4 relative to wavelength λ corresponding to transmission frequency (central frequency) f0, which is a high frequency. In this case, the tip end of each of bent portion 2x, 3x is an electric short-circuit end, and each bending point function as an electric open end.
In a balun like this, balanced outputs from amplifier 7 in mutually opposite-phase, using a ground potential as a reference, are applied to balanced microstrip lines 2, 3 (i.e., signal lines 2, 3) of the high-frequency balun. Then, the balanced outputs in mutually opposite-phase travel in balanced microstrip lines 2, 3, using ground potential 5 as a reference potential. Since tip end of each of bent portions 2x, 3x is an electric short-circuited ends and each bending point functions as an electric open end, standing waves W1, W2 in mutually opposite-phase with electric lengths of λ/4 are generated in both bent portions viewed from each bending point such that the bending points are maximum voltage displacement points and the tip end points are minimum voltage displacement points (i.e., zero voltage points). Incidentally, amplifier 7 further includes an unbalanced input terminal, a power source terminal connecting to power source Vcc, and a ground terminal connecting to a ground potential point.
Then, since each bent portion 2x, 3x of balanced microstrip lines 2, 3 are mutually close to unbalanced microstrip line 1, both are electromagnetically coupled. Therefore, standing wave W of electric length of λ/2, which regards both ends as maximum voltage displacement points in mutually opposite-phase, is induced in unbalanced microstrip line 1, while center point P between bending points of balanced microstrip lines 2, 3 is approximately regarded as a reference point (i.e., null potential point). With this arrangement, in unbalanced microstrip line 1, the high-frequency wave component in unbalanced mode between signal line 1a and ground conductor 5 travels toward the left end side of unbalanced microstrip line 1, while the opening end of unbalanced microstrip line 1 (right end in FIG. 1A, maximum voltage displacement point) is regarded as a starting point. Then, for example, coaxial cable 8 is connected to unbalanced microstrip line 1, and the high-frequency wave is transmitted to coaxial cable 8 in unbalanced mode.
In this way, in the above high-frequency balun, each of bent portions 2x, 3x of balanced microstrip lines 2, 3 is set to a length of λ/4 relative to wavelength of λ corresponding to transmission frequency f0, and then a standing wave of λ/2 is generated. In other words, bent portions 2x, 3x are resonant with transmission frequency f0 corresponding to standing wave of λ/2. Then, bent portions 2x, 3x are electromagnetically coupled to unbalanced microstrip line 1, and transmission frequency f0 in unbalanced mode is obtained. Specifically, in the high-frequency balun using the microstrip line coupling line, the balanced mode is converted to the unbalanced mode and vice versa using resonant phenomenon, and transmission frequency f0 is obtained. Therefore, as shown in FIG. 2, single peak characteristic (curve L) is obtained after conversion, while transmission frequency characteristic (curve K) having a linear property is provided before conversion, and there is a problem in that the band width of transmission frequency f0 is narrowed.
Further, as shown in FIG. 3, when microstrip line 15 is merely branched in parallel, and one branch microstrip line 15a is made longer (or shorter) than another branch microstrip line 15b by λ/2 with respect to transmission frequency f0, the high-frequency wave component in balanced mode in mutually opposite-phase can be obtained. However, in this case, since only transmission frequency f0 corresponding to wavelength λ is in opposite-phase, it causes a narrow band characteristic.